Graphene is a two-dimensional crystal of carbon formed from a monoatomic layer of sp2 hybrid carbon atoms (having the structure of a benzene ring corresponding to hexagonal cells), graphite being formed from graphene sheets having a thickness corresponding to the size of a carbon atom.
Notably, the article “The Rise of Graphene”, Nature Materials, Vol. 6, page 183, 2007 by A. K. Geim and K. S. Novoselov has shown an atomic plane of sp2 hybrid carbon atoms and the various structures that may stem therefrom: fullerenes, carbon nanotubes and graphite that are illustrated in the present application by FIGS. 1a, 1b, 1c and 1d respectively.
Although evoked in the formation of fullerenes, carbon nanotubes and graphite, graphene had never been isolated and studied. Its stability has even been contested, all the crystals having a tendency of having been thermodynamically unstable at small thicknesses (the surface atoms less well bonded become predominant in relation to those of the volume).
The first graphene films were isolated in 2004 (K. S. Novoselov et al., “Electric Field Effect in Atomically Thin Carbon Films”, Science, Vol. 306, page 666, 2004) and have proved to be remarkably stable. These films are obtained by “exfoliating” blocks of a graphite called HOPG (Highly Ordered Pyrolytic Graphite), which is a commercial material. Graphite is a lamellar material formed from stacks of graphene sheets, and the bonds between horizontal planes are weak. Exfoliation consists in removing graphene planes using adhesive tapes. The method is simple and not very reproducible, but it does make it possible to obtain graphene sheets measuring of the order of 10 to a few tens of μm in one of the dimensions.
Obtaining these first graphene sheets has made it possible to characterize them and to show that graphene is a stable, highly conductive ambipolar material (i.e. able to exhibit two types of conduction, by holes or by electrons; it is in fact a zero-gap semiconductor) having high carrier (electron or hole) mobilities (of the order of 10 000 to 100 000 cm2/Vs at low temperature).
Very advantageously, graphene may thus be applied, on the one hand, to the fabrication of thin-film transistors (provided that the width of the strips is precisely controlled so as to open an energy gap in the band structure of the material) while on the other hand it makes it possible to provide thin transparent metal layers as a replacement for transparent conductive oxides (i.e. ITO or indium tin oxide) in flat screen displays, in solar cells and in general in all applications requiring a transparent conductor. This material has proved to be beneficial for films having up to about four graphene monolayers (a material called FLG or “few-layers graphene”). This advantage is a major advantage in the context of seeking to replace ITO because of the rarity and therefore the costliness of indium.
However, for a practical use it seems difficult to employ the exfoliation method, as this does not enable the thickness (i.e. the number of graphene layers) or even the geometry of the deposit, to be precisely controlled.
Various preparative methods have been suggested, such as for example the partial oxidation of graphite in acid medium, enabling it to be exfoliated in liquid medium. The graphene can then be put into aqueous suspension and deposited, for example by filtration, by spray coating or spin coating but with the problem that the thickness of the layers obtained is not uniform.
To obtain acceptable electrical conductivity values, it is then necessary to carry out a chemical reduction (to remove the intercalated oxygen). A process of this type, which is nevertheless very complex, has been described in the article by G. Eda et al., Nature Nanotechnology, Vol. 3, page 270, May 2008.